1. Field of the Invention
The present invention relates to temperature measurement using pyrometry, and more specifically, it relates to the measurement of temperature and emissivity using single-fiber multi-color pyrometry.
2. Description of Related Art
Radiation thermometry is a common non-contact method of measuring temperature. Planck""s Law states that the spectral radiance of a target is a function of its temperature. Hence, the signal produced by a detector that is sensitive to all or part of the radiated thermal spectrum will be related to the temperature of the target. However, the spectral radiance of a target is also governed by its emissivity. Consequently, the signal will depend on the emissivity of the target as well as its temperature. Furthermore, the measured radiance may be comprised of unwanted ambient system radiance as well as the desired target radiance, and may be weak compared to the detector background level.
Currently, many temperature sensing devices employ the method of two-color pyrometry to eliminate the effect of unknown or varying emissivity. Two color pyrometers sample the target radiance in two different spectral regions, and calculate the true temperature and/or emissivity using various algorithms. Several techniques for separating the target radiance into two spectral regions have been identified. One technique involves a beamsplitter to direct the incident radiation into two paths, each of which contains a detector. A second method incorporates a rotating filter wheel composed of two different filters and a single detector. Another method uses a two-color detector consisting of two different active regions. As with any radiation thermometer, the spectral characteristics of the optical components determine the useful temperature range of the device.
The systems developed by X. Maldague, et. al. [Opt. Eng. 28(8):872-80] and U. Anselmi-Tamburini, et. al. [Rev. Sci. Instr. 66(10):5006-10] both employ beamsplitters to separate the incident radiation into two paths, each of which contains a detector. The device patented by K. Crane, et. al. [U.S. Pat. No. 4,470,710] employs a wheel composed of alternating infrared filters of different bandpass and a single detector. These systems are suited only to high-temperature measurement. High-temperature measurement methods require large target signals that are generally much stronger that any ambient radiation. In general, simply replacing the optical components in high-temperature devices with longer wavelength components will not provide a clear signal for low-temperature measurement, because the necessary means of distinguishing the small target signal from the ambient noise is missing.
The system developed by O. Eyal and A. Katzir [Opt. Eng. 34(2):470-3] exhibits state-of-the-art technology for remote low-temperature two-color pyrometry. A single silver halide optical fiber collects radiation emitted by a target and transmits it to an optical chopper which modulates the radiation for lock-in amplification before it is focused onto a single two-color mid-infrared detector. The side of the chopper facing the detector is made reflective to stabilize the lock-in signals by one of two methods: either a reference blackbody of controlled temperature is positioned such that the detector alternately xe2x80x9cseesxe2x80x9d it and the target, or a black line is drawn on the chopper blades to control its emissivity. This device offers several features. First, the use of a single collection fiber ensures that each spectral region is comprised of radiation emitted by the same spot on the target, which, when coupled with the two-color principle, minimizes the influence of the area of the spot (i.e. fiber tip to target distance). Second, lock-in amplification enables recovery of small signals generated by low-temperature targets. Third, the reflective chopper provides a means of lock-in signal referencing. Fourth, the two-color mid-infrared detector incorporates the two active regions in a single element. If the spectral sensitivities of the optical components were chosen differently, high-temperature measurements could theoretically be made using the same method.
There is a need to perform color-temperature measurements using a fiber-based system in which the detected radiation is collected by a single fiber, and the radiation is detected in two or more wavelength bands. Single fiber collection eliminates the need to align multiple fibers to a common spot on the target. The method of Eyal and Katzir allows such a measurement technique using an integrated two-color detector system, but its extension to multiple wavelength bands is limited by detector technology. The present invention may be extended to multiple bands, and does not rely on sophisticated detector arrangements. Like the above low-temperature device, this method uses a single optical fiber to ensure that the radiation collected in each spectral band originates from the same spot on the target.
It is an object of the invention to noninvasively measure temperature and emissivity of a target in real-time.
It is another object of the present invention to noninvasively monitor the surface temperature and emissivity of biological tissues before, during, and after laser irradiation.
It is also an object of the invention to provide a feedback loop to control laser power output during irradiation of biological tissues for laser tissue welding.
This invention is an apparatus and method for non-contact real-time true temperature and emissivity measurement. A single fiber is used to couple the spectral radiance from a spot on the target into a multi-color pyrometer, which consists of a reflective chopper, two or more detectors possibly of different spectral bandwidth with or without filters to limit the wavelength regions detected, optics to direct and focus the radiation onto the detectors, lock-in amplification, and a computer algorithm based on previous blackbody calibrations. Among the advantages of this method are (i) the radiation signal collected by the fiber is independent of the fiber to target distance (for a target of uniform temperature over the observed surface area); (ii) the radiation observed through all channels originates from the same geometric region on the target (which is not true when a different fiber is used to collect radiation for each channel); (iii) the measured temperature is independent of the target emissivity and corresponds to the true target temperature if the emissivity is independent of wavelength within the measurement band; and (iv) target emissivity can be determined.
The radiation transmitted by the fiber is either passed or reflected by a reflective chopper, thus modulating the radiation for lock-in amplification and splitting the radiation into two or more paths simultaneously. Each path consists of a detector and focusing optics, and possibly optical filters to limit the spectral bandpass of the radiation incident on the detector. If no filters are used, then the detectors must all have different spectral sensitivities. The use of multiple paths as provided by the reflective chopper allows for the addition (or subtraction) of detectors, making the system a multi-color, as opposed to a strictly two-color, pyrometer. Furthermore, separate paths offer more freedom in choosing detectors and filters. The reflective chopper, which splits the incident radiation into two or more paths, while simultaneously modulating the radiation for lock-in amplification, eliminates the need for additional components to split the radiation. For the case of a two-color system, the chopper could be a rotating planar disk with alternating reflective and transmissive veins. It may also consist of a multi-faceted reflective surface to reflect the beam into various paths. The chopper may also be constructed with other types of mechanical devices commonly used for chopping light signals, such as a resonant arm device with a multifaceted optic.
The detection system measures the radiation intensity emitted by the target in each of the spectral bands using lock-in amplification. The lock-in signals are proportional to the difference between the target signal and a background signal originating from the background radiation field within the detection system. (In the chopper-closed position, the radiation striking the detector originates from the background only while in the chopper-open position the radiation striking the detector originates from the target as well as the background.) Calibration of the system requires measurements of the signal levels for each channel as a function of target temperature, using a target with known emissivity (usually a blackbody with emissivity equal to 1). These calibration curves can be fit accurately with a simple parametric function and used for numerical solution during measurements of target signals. Variations in the background level arising from temperature drifts may be compensated by independently measuring the background temperature (with a thermocouple) and applying temperature-dependent corrections to the measured signal levels.
A computer algorithm computes the true temperature and emissivity in real time using the resulting signals and the previous blackbody calibration. For a measurement system operating at near-ambient or lower temperatures, a thermocouple mounted inside the closed system is needed to monitor fluctuations in the background temperature, allowing for dynamic compensation of the background level. There is no need for an additional temperature-controlled blackbody or other referencing means. The resulting signals are converted to temperature and emissivity based on a temperature-controlled blackbody calibration. This method can be tailored to various temperature ranges by selecting the appropriate spectral characteristics for the optical components.
The temperature range of the pyrometer system is determined by the spectral characteristics of the optical components. The spectral characteristics of the fiber, detectors, and filters govern the useful temperature range, as implied by Planck""s Law. The spectral bandwidths of the detectors and any filters must be at least partially included in the radiated thermal spectrum. Furthermore, the bandwidth of the fiber must at least partially include the spectral regions sensed by the detectors. The wavelength region sensed by one of the detectors relative to that of the other also affects the useful temperature range. For example, for two-color pyrometry, if the calibration yields a ratio that is nearly independent of the temperature or, similarly, the two detector calibration equations are degenerate or without a simultaneous solution, then accurate measurement of the temperature and emissivity is not likely.
Dynamic room temperature-regime measurements for a target of unknown or changing emissivity can be made using this multi-color system, with mid-infrared optical components. Such a system has been developed that incorporates the two-color principle using a single hollow glass waveguide and thermoelectrically cooled HgCdZnTe photoconductors along with the reflective chopper/lock-in amplification set-up. A 700 xcexcm-bore hollow glass waveguide coated with a dielectric layer on the inner surface, capable of transmitting wavelengths greater than 2 xcexcm, is used to collect the spectral radiance from the target. A gold-coated planar chopper is used to split and modulate the incident radiation. Two 128.8 mm-radius gold-coated spherical mirrors focus the radiation onto their corresponding HgCdZnTe photoconductors. The spectral bandwidths of the photoconductors are 2-12 xcexcm and 2-6 xcexcm, respectively. This particular configuration permits measurement of temperatures from below room temperature to above 200xc2x0 C. Radiation from a target is collected via the single 700 xcexcm-bore hollow glass optical fiber coated with a dielectric layer on the inner surface, simultaneously split into two paths and modulated by the gold-coated reflective chopper, and focused onto the two thermoelectrically-cooled mid-infrared HgCdZnTe photoconductors by the 128.8 mm-radius gold-coated spherical mirrors. The modulated detector signals are recovered using lock-in amplification. The two signals are calibrated using a blackbody (emissivity equal to 1) of known temperature, and exponential fits are applied to the two resulting voltage versus temperature curves. Using the two calibration equations, a computer algorithm calculates the temperature and emissivity of a target in real time, taking into account reflection of the background radiation field from the target surface.
The present invention may be used to noninvasively monitor the surface temperature and emissivity of biological tissues before, during, and after laser irradiation. It may be used in a feedback loop to control laser power output during irradiation of biological tissues for laser tissue welding. The invention could be used to noninvasively measure temperature and emissivity of a target in real-time.